A process for preparing polycarbonate by copolymerizing epoxide and carbon dioxide with a complex containing onium salt as a catalyst, and a process for regenerating the catalyst by isolation and recovery of the same from the mixed solution of the product copolymer and the catalyst are disclosed by the present Applicant in Korean Patent Registration Nos. 10-0853358 and 10-0981270.
It is a rare case that a metal catalyst for polymerization is not isolated from the final product (as is the case for Ziegler-Natta catalyst). But the catalyst must be removed or recovered on the following reasons.
In a final stage for preparing polycarbonate through copolymerization of epoxide and carbon dioxide by using a complex containing onium salt as a catalyst, the catalyst is directly chemical-bonded with the copolymer chain at the terminal group of the long copolymer. As time goes by, while the catalyst is not isolated as such, the copolymer is degraded into carbon dioxide and cyclic propylene carbonate (CPC), thereby reducing molecular weight of the copolymer and deteriorating all physical properties accordingly, so that the product value of the copolymer is lowered.
If a ligand containing chromophore or a transition metal complex is not completely removed from the product, the final product may be colored to cause problems in product quality or product value. Further, applications of the copolymer might be restricted due to toxicity of the transition metal.
Since the catalyst for copolymerization of carbon dioxide/epoxide, the subject matter of the present invention, is comprised of expensive metal as well as plenty of high-price ligand, in particular, the used catalyst must be not only removed but recovered from the viewpoint of product economy.
In order to isolate or remove the catalyst used for polymerization, commonly utilized is alumina, silica gel, ion exchange resin, or the like (Prog. Polym. Sci. 2004, 29, 1053). More commonly used method is to remove the catalyst by direct interaction with functional groups such as —OH and —SH being present on the surface of an adsorbent. Silica or alumina, however, generally has low rate for catalyst removal, so that an excess amount should be used in order to remove the catalyst up to sufficient level. Thus, high material cost as well as the process cost (for solvent used, or for pressure loss) are required. It is necessary to consider economy of the subjective process by searching for an adsorbent with high efficiency for catalyst removal in a small amount, and a process for regenerating it.